Capacitive-coupling control system



Feb. 14, 19 28. 1 658305 L. L. JONES CAPACITIVE COUPLING CONTROL SYSTEM Original Filed Dec. 15, 1922 iL za /6' /Z 1} 15 T 18 f? INVENTOR LE5 719? 1 via/via TORNEYS Patented Feb. 14, 1928.

PATENT QFFEQE.

LESTER L. JONES, OF ORADELL, NEW JERSEY.

CAPAGITIVE-COUPLING CONTROL SYSTEM.

Original application filed December 15, 1922, Serial No. 607,046. Divided and this application filed March 15, 1927.

This invention relates to a thermionic tube circuit system, and more particularly to a tube circuit system wherein the thermionic tube functions as a relay or amplifier of en- 5 ergy transmitted from the input or grid circuit to the output or plate circuit; and has special reference to the provision of a tube circuit-system in which the re-transfer or feed-back of energy from the plate to the "rid circuit due to the capacitive coupling between the plate and grid is made control-v lable and may be efiectively minimized or eliminated.

This application is a division of my co- 5 pending application Serial No. 607,046, filed December 15, 1922.

In the use and operation of thermionic tubes as employed for example in the transmission and reception of audio and radiant energy, it is well known that due to the unshieldable capacity between the plate and grid of the tube, a re-transfer or feed-back of energy from the tube to the input circuit takes place. This energy or power feedback is highly objectionable, especially in relay, amplifying or modulating circuits, be cause it gives rise to distortion of the input volt-age power wave form and to incipient or sustained oscillations in the grid circuit.

To reducethe feed-back or re-transferrance of energy to avoid these defects, it has been common engineering design to construct the tube with a minimum of capacity between the 'grid and plate.

.pacity between the plate and the grid, however, merely reduces but does not eliminate the re-transferrence of energy, the reduction of the capacity being in itself disadvantageous, since it reduces the efficiency of the 40 tube system.

When the tube is employed as an audio frequency amplifier, it is found that when the amplifier is made eflicient, that is, to amplify 20 or more times audibility per stage,

the tone quality is defective because an abrupt strong current pulse will set the amplifier system into an oscillation which is unduly prolonged by the re-transfer of energy from the plate circuit to the grid circuit throu h I the grid-plate tube capacity. If the amp i- This reduction in theca-' Serial No. 175,512.

An expedient for minimizing this effect has been to shunt the output impedance of the output circuit by a large condenser, but this expedient has the disadvantage of suppressing the higher tone frequencies. In order to ob viate this objection, it has been also attempted to shunt a high resistance around the input circuit from the grid to the filament, this being objectionable, however, in that the efficiency of the system is materially reduced. When the thermionic tube is utilized in the transmission or broadcasting of radiant 55 energy, the energy or power feed-back due to the capacitive coupling between the plate and grid circuits is also found to be decidedly deleterious in its effects. The theoretically best type of tube transmitter from the operating View point is the master oscillator or separately excitated grid system. This system has not worked out practically in the past, mainly because of the energy transfer back to the master oscillator through the tube capacity coupling which in the ease of transmitting tubes is very large. This oscillating energy feed-back causes distortion of the master oscillation or sustains it to; the point where no minimum appears. To avoid this, it has been necessary to so damp the master oscillator or the exciting grid circuit that very little amplification is obtained. This necessitates the use of master oscillation circuits of the approximate power of the antenna circuit, or many stages of amplifi cation, the use of either resulting in such low over-all etliciency that the system is impractical for all except very low power transmitters where the power cost iSmIlOt an economic factor. A prime desideratum of my present invention comprehencls theprovisionof a tube circuit system in which the feed-back or retransferrence of energy or power from the plate to the grid circuit resulting from the capacitive coupling between the circuits due tothe grid-plate capacity is effectively controlled so that the same may be reduced to a value below that resulting from the gridplate capacity of the tube and which may be reduced to a minimum or entirely eliminated. By the elimination of this energy feed-back the tubes need not be designed with a minimum of capacity as is the case in the present .105 practice, and the tubes may be designed to produce a maximum efiiciency 1n the use and operation thereof. -When employed as an audio frequency ampllfier, the above-mem tioned expedients need not he resorted to,

and the amplifying system may be made to have the highest etficiency without that howling or distortion which results from the energy transfer back to the input circuit. When employed in transmitting systems, the

r tube circuit system of my invention ma ergy-transfer device with all its consequent advantages when employed in audio and radio transmitting and receiving systems.

Looked-at from a broader aspect. my present invention contemplates the provision of means for adjusting the capacitive coupling between the grid and plate of the tube to points below the natural or grid-plate capacity ofthe tube down to a minimum so that the energy feed-back may be reduced to magnitudes or values below that resulting from the 'rid-plate tube capacity. T his latter control 1s found desirable in a number of circuits, as for example in capacitive convention may be said to include, besides the.

. pling re-generative circuits such as the Arm-. strong tunedwmg and grid circuit. this con-- trol permitting regeneration at a value below the natural tube capacity. By means of the capacitive coupling control of my invention, moreover, the efi'ect of the natural tube capacity may be not only reduced or eliminated, but'may be reversed so as to permit de-generative effects.

The principal objects of my present inprovision of a capacitive feed-back control and eliminator for thermionic tubes as above referred to; the more specific provision of a tube circuit system'in which the charge carried to the grid from the plate through the capacity therebetween may be neutralized in whole or in part, thus reducing to any desired degree or completely eliminating the l e-transfer of potential and the retransfer or feedback of energy; the further provision of a tube circuit system in which a charge is impressed on the grid which may be less than, equal to or greater than in magnitude but opposite in sign to the charge impressed thereon or carried thereto from the plate circuit; the further provision of a tubecircuit system in which a charge is impressed on the grid which is equal in phase and opposite in sign to the charge carried thereto from the output circuit through the gridplate capacity; and the still further provi-. sion of a tube system of this nature in which the neutralizing charge impressed on the grid is caused to vary in correspondence to the variations of the charge carried to the grid from the plate in the normal use and operation of the tube.

To the accomplishment of the foregoing and such other objects as may hereinafter appear, my invention consists in the elements and their relation one to the other. as hereinafter particularly described and sought to be defined in the claims. reference being bad to the accompanying drawings which show a preferred embodiment of my invention. and in which:

Fig. 1, is a wiring diagrammatic view of the thermionic tube circuit system provided with an inductively coupled trans'l mmerin the plate circuit and showing my invention applied thereto.

Fig. 2 is a wiring diagrammatic view of a thermionic tube circuit system provided with a choke coil in the plate circuit and showing a modification ot my invention. applied thereto,

Fig. 3 is a rc-arrangemenl of the circuit system of Fig. l exemplifying the effect. of the same as snnulating a balanced bridge,

Fig. 4 is a wiring diagrammatic view of another type of circuit showing my invention applied thereto, 1

Fig. 5 is a wiring diagrammatic view showing a modification of my invention applied to a circuit of the type shown in Fig. 1, and

Fig.

of my invention.

Referring now more in detail to the draw-' 6 is a cross-sectional view of a control means wlnch may be. employed as part connected in parallel with the inductor 14,

the inductor and condenser being connected to the grid and the filament by means of conductors 16 and 17 respectively. The plate 13 is included in a plate or output circuit provided with an external or output impedance 18 connected at one end by means 'of the conductor 19 to the. plate 13 and connected at the other end by means of the conductor 20 to the positive terminal of the bat tery 21, the negative terminal of which is connected to the filament 10 as clearly shown in the drawings. The output impedance 18 may, as shown, comprise the primary of an air core transformer, and is coupled to the secondary 22 of the said transformer, the said secondary receiving at resonance the amplified energy for further amplification if desired, resonance being obtained by the condenser 15".

As heretofore mentioned, due to the normally unshieldable capacity between the plate 13 and the grid 12 of the tube, some of the energy in the output circuit is objectionably re-transferred or fed back to the input circuit, the grid 12 being charged by the plate 13 through the capacity therebetween. My present invention contemplates broadly the provision of means for controllably reducing or entirely eliminating the effect of the charge carried from the plate to the grid, the power re-transferrence being, more specifically, controllably reduced or eliminated by neutralizing the feed-back of potential or neutralizing in part or whole, the charge carried from the plate to the grid. This result I preferably accomplish by providing means for impressing on the grid 12 a charge which may be less than, equal to or greater than in magnitude but oppositeiin sign to the charge carried thereto from the plate, and to this end there is provided means for creating a potential opposite to the potential on the plate and impressing the said potential on a capacity means connected to the grid 12. Where feedback is desired to be entirely eliminated, the product of the said potential and the capacity is made equal to the charge transferred to the grid from the output circuit. One manner of accomplishing this is exemplified in Fig. 1 of the drawings, which shows my invention applied to an amplifying circuit and in which the potential created is made equal in phase and amplitude and opposite in sign to the potential on the plate, and the capacity connected to the grid is preferably made 'equal to the measured capacity between the plate and the grid. To these ends, I provide, as shown in Fig. 1 of the drawings, an additional impedance 23 comprising a coil wound in the same direction asthe output impedance coil 18 and having a number of turns equal to the number of turns in the winding 18, the said additional impedance being connected at one end to the conductor 20 andbeing connected at its other end by means of a conductor 24 to a condenser 25 which is connected as shown to the high potential side of the input circuit, the said condenser 25 being made equal to the measured capacity between the grid 12 and the plate 13 and the capacity between the conductors 'or leads connected thereto (of the order of 6 mmf.s for present-day receiving tubes). The additional impedance 23 is preferably closely coupled tothe output impedance 18, and this impedance serves to produce the potentialequal in phase and amplitude and opposite in sign to the potential on the plate 13, these potentials being taken with reference to the filament potential. By impressing this potential on the condenser 25, it will be apparent that the grid 12 is supplied with an electric charge which is equal and opposite to the charge produced thereon as the result of the capacitive coupling between the in put and output circuits neutralizing the said latter charge, and it will be further noted that by virtue of the balancing or equalizing impedance and capacity 23 and 25 respectively, the neutralizing charge varies in correspondence with the varying charge on the plate in the operation of the circuits, so that the energy feed-back is eliminated for all variations .of the grid and plate circuits, and that when once adjusted, the equalizing circuit will eliminate feed-back at all frequencies.

The effect of the charge neutralizing circuit of my invention may be seen from a consideration of Fig. 30f the drawings,

which comprises a re-arrangement of the circuit system shown in Fig. 1 and an arrangement of the same in a simulated balanced bridge formation: The condenser 25 in one arm of the bridge is made equal to the natural tube capacity 26 between the grid and plate in the corresponding arm of the bridge, and the additional impedance 23 is made equal to the output impedance 18. To minimize in a practical way any unbalance that results from the plate com- .ponent of voltage-in the plate circuit, the

output impedance and the added impedance 2 are closely coupled with a coupling, however, substantially less than unity. By a coupling substantially less than unity, I desire it to be understood that I mean that range of couplings that can be obtained with air cored transformers; and as shown in the drawings, the output impedance 18 and the added impedance 23 are air core coupled. In most practical cases where the output impendance forms the primary of a trans former for the further transfer of energy, as shown in Figs. 1 and 3 of the drawings. it is suflicient if the output impedance and the added impedance are separately and closely coupled to the secondary ofthe transformer. Thus the output impedance 18 and the added impedance 23 as shown in both Figures 1 and 3 of the drawings are separately and closely coupled to the secondary 22 of the transformer; and as will be seen by reference to these Figs. 1 and the outputand added impedances 18 and 23 are each coupled similarly or equally to the secondary coil 22 of the coupling transformer.

Referring now to Fig. 2 of the drawings, I showa modification of my system-employed with tube circuits provided witha choke coil in the plate circuit, and wherein nected and coupled to the choke coil im-.

pedance 18 I provide the additional impedance 29 for creating a potential opposite in direction to the potential on the tube plate 13'. The additional impedance 29 may differ in size from the output impedance 18, and as shown may be made smaller than v the output impedance. The reverse potential creating impedance 29 is connected by Ineansof the conductor 30 to a condenser 31 which in turn is connected to the high potential side 16 of the grid circuit: and the said condenser 31 may also ditier in magnitude from the capacity between grid 12 and plate 13, the capacity of this condenser 31 and the impedance of coil 29 being, however, so designed that the product of the capacity and the potential created by the impedance is substantially equal and opposite to the transferred charge on the grid 12. This system shown in F ig. 2 is not as eflicient as the balanced system shown in Figs. 1 and 3 of the drawings, but may be employed for obtaining substantial elimination of the energy feed-back wherever a full elimination there of is desired.

As heretofore mentioned, my invention may be applied to tube circuits where it is found desirable to adjust the grid-plate capacitive coupling to a point between thenatnr'al capacity of the tube and a zero value, and-if desired to adjust the same in the reverse direction. In Fig. 4 of the drawings, this application of my invention is e emplified as applied to the Armstrong tun d wing and grid circuit; and referring to Fig. 4, I show the said wing and grid circuit generally designated as 32, which circuit is modified by removing the conductor 33 (shown in dotted lines) which connects the wing circuit 34 to the telephone receiver 35, and by tapping the midpoint of the inductance 36 of'the wing circuit and connect+ ing the same by means of the conductor 37 to the telephone receivers 35, the wing circuit being further connected to the high potential side 38 of the grid circuit by means of the conductor 39, condenser 40 and conductor 41. said condenser 40 being made variable as hereinafterto be described in detail so as to vary the charge impressed upon the high potential side-0f the grid circuit. In the unmodified Armstrong tuned wing and grid circuit, no advantage has been found in the use of good or high power amplifying tubes. This is due to the fact that when the wing circuit is tuned to the incoming wave, the energy feed-hack through the grid-plate. capacity is so strong as to set the grid circuit into oscillation. When such a tube is used, it is therefore necessary either to detune the plate circuit considerably, thus losing a large part of the selectivity of this system, or to reduce the filament or plate voltage so as to reduce the tube amplification. This latter method reduces the signal intensity although it retains pressed on the high potential side of the I grid circuit by means of the equalizing circuit of .my invention, the condenser connected to the high potential side of the grid circuit may be. made variable. For the purpose of preventing the variation of capacity of 'this condenser from varying the tuning characteristics of the grid or input circuit, I have found it desirable to provide means for varying the capacity of the input circuit simultaneously and-in correspondence with the varying of the capacity in the equalizing circuit, the construction provided to this ,end being shown in Fig; 5 of the drawings. Referring to this figure, which depicts the tube circuit shown in Fig. 1 of the drawings, like parts-0f which are indi cated by primed reference characters, con'-' necting the equalizing impedance 23' of the equalizing circuit to the high potential side of the-grid circuit I provide a variable condenser generally designated as 25 which in the preferred construction includes three spaced plates a, b, c, the plate 0 being connected by the conductor 24 to the equalizing impedance 23', the plate a being connected to the grid by means "of the conductor 42, and the plate I) being connected to the filament end of the grid circuit which-is at ground potential by means of the conductor 43. The plate a is made adjustable and is movable in opposite directions as indicated by the arrows shown in the figure, with the construction such that as the plate a is moved to the right, the capacity between the plates a and b is increased and the capacity between the plates a and c is correspondingl v decreased, and as the plate a is moved to the left the capacity between the plates and c is increased and the capacity between the plates a and b is correspondingly decreased in inverse ratio. In view of the negligible impedance of the coil 23' as compared with the impedance of the grid coil 14', the change m capacity between the plates a and-b isnormally efi'ective for modifying the capacity of the grid circuit, and by the construction provided the grid circuit capacity may be maintained constant during adjustment of the plate a and during the variation of the-charge nnpressed upon the grid 12. l

The three-plate condenser of my invention may constructionally take the form shown in Fig. 6 of the drawings, in which the three plates comprise cylinders or tubes a, b and c, the tubes 6' and 0' being fixed or stationary and the tube a being telescopically movable in opposite directions within the tubes 6 and c. The tube 12 may be fixedly carried by a collar 44 and secured thereto as by the screw 45, the said collar being threadedly received by a threaded bushing 46 which is stati'onarily mounted on a panel.

board 47 the panel-board being secured between the shoulder 48 of the bushlng 46 and the collar 44. Fitting within the tube '7) I provide a tube of insulatmg material 49 such as hard rubber on which the tube 0' is fixed by means of the screw 50 and within which the tube (1 slides.

For telescopically or slidably moving the plate-tube a and indicating its position there is provided a spindle 51 which carries the tube a at one end and which is threaded the greater part of the length at the other end as shown at 52 for threadedengagement with atapped member 53 attached to an insulating dial member 54 which carries a pointer 55. The spindle 51 is longitudinally recessed as at 56 for receiving a pin 57 fitted into the bushing 46, this to the end of constraining the spindle to move axially only'upon rotation of thedial 54. With this construction, it will be apparent that upon rotation of the dial 54 in opposite directions, the spindle 51 and the condenser tube-plate a will be moved in opposite directions for varying the capacity between plates :1 and b in inverse relation to that between the plates a and c. It will be further seen that when plate I) is connected to ground while plates at and c are connected respectively to the grid and plate coil, the plate 6' provides a ground shield for the small unit condenser ac, preventing that interference presented by the hand or body of the operator.

The use and operation of my capacitive feedback control or eliminator and the control condenser therefor will in the main be fully apparent from the above detailed description taken in connection with the many possible applications thereof. It will be seen that with my invention I am enabled to control the efiect 'of grid-plate capacitive coupling and vary the same to points below the natural capacity of the tube, the capacitive feed-back effect being controllable so as to permit reduction to produce desired regeneration, to permit of complete elimination, to produce an absolutely one-way energy transfer device, and so as to ermit reversal where degeneration is desire to be efie'cted. It will also be seen that in my capacitivecoupling control the grid-plate capacitive coupling may be varied without altering the period of the input circuit.

While I have shown my device in the preferred forms, it will be obvious that many changes and modifications may be made in the structure disclosed without de arting.

from the spirit of the invention, de ed in the following claims.

system, said auxiliary. coil being coupled to the first coil with a coupling less than unity,

and said auxiliary coil and first coil being seplarately and closely coupled to said second 001 Y 2. A tuned radio frequency multi-stage i amplifier comprising a plurality of stages,.

each stage having an electron discharge device containing a grid and plate circuit, and mechanism in each stage for neutralizing the capacity coupling between the grid and plate circuits of such stage due to the capac- 1ty between the grid and plate electrodes, 7

said neutralizing mechanism for each stage comprising a coil connected between the plate and the filament system of sucl; stage, and an auxiliary coil anda neutralizing capacity connected in series between the grid and the filament system of such stage, said auxiliary coil being closely coupled electromagnetically to the first coil but with a coupling substantially less than unity, and said 4 coil and auxiliary coil being coupled closely to the gr1d circuit of the next adjacentstage,

and means for tuning the grid circuit of 'at least one of said stages.

3. A tuned radio frequency multi-stage amplifier comprising a plurality of stages,

each stage having an electron discharge de- 'vi'ce containing a grid and plate circuit, and

mechanism in each stage for neutralizing the capacity coupling between the grid and plate circuits of such stage due to the capacity between the grid and plate electrodes, said neutralizing mechanism for each stage comprising a coil connected between the plate and the filament system of such stage, an auxiliary coil and a neutralizing capacity connected in series between the grid and the filament system of such stage, said auxiliary coil being closely coupled electromagnetically to the first coil but with a coupling substantially less than unity, and said coil and auxiliary coil being. coupled equally to the grid circuit of the next adjacent stage, and

means for tuning the grid circuit of at least one of said stages, the auxiliary coil and the 10 first coil having aratio of turns equal to the ratio of the coupling capacity to the neutralizing capacity.

4. A tuned, radio frequency multi-stage amplifiercomprising a plurality of stages,

1 each stage having an electron discharge de vice containing a grid and plate circuit, and

mechanism in each stage for neutralizing the capacity coupling between the grid and plate circuits of such stage due to the capac- 'ity between the grid and plate electrodes, said neutralizing mechanism for each stage comprising a.coil connected between one of the electrodes and the filament system and an auxiliary coil and a neutralizing capacity connected in series between the other of these electrodes and the filament system, said auxiliary coil being closely coupled electromagnetically with a coupling substantially less than unity to the first coil and said coil.

and auxiliary coil being coupled closely to the adjacent circuit of the stage next adjacent thereto, and saidcoil and auxiliary coil having aratio of turns equal to. the ratio of the coupling capacity to the neutralizing capacity, and means for tuning the grid circuit of at least one of said stages. a v

5. A-tunedradio frequency multi-stage .amplifier comprising'a plurality of stages,

each stage having an electron discharge'device containing a grid and plate circuit, and mechanism in each stage for neutralizing the capacity coupling between the grid and plate circuits of such stage due to the capacity between the grid and plate electrodes, said neutralizing mechanism for each stage comprising a coil connected between one or the electrodes and the filament system and an auxiliary 'coil'and-a neutralizing capacity connected in series between the other of these'electrodes and the filament system,-said auxiliary coil being closely coupled electromagnetically with a coupling substantially less than unity to the coil, and said coil and .between the grid and platecircuits of such stage due to'the capacity between the grid and plate electrodes, said neutralizing mechanism for each stage comprising a capacity connected between a grid of the electron discharge device and a coil coupled to the primary of the transformer of that stage with a coupling substantially less than unity, said 0011 and transformer primary being equally-- coupled to the secondary of the transformer, and means for tuning at least one of the grid circuits of said stages.

7. A tuned radio frequency multi-stage amplifier comprising a plurality of stages, each stage having an electron discharge device containing a grid and plate circuit, a transformer in each stage for coupling one stage to the next,and mechanism in each stage for neutralizing the capacity coupling between the grid and plate circuits of such stage due to the capacity between the grid and plate electrodes, mechanism for each stage comprising a capacity connected between a grid of the electron'discharge device and a coil closely coupled'to the primary 'of the transformer of that stage with a coupling less than unity, said coil and transformer primary being separately and closely coupled to the secondary of the transformer, and means for tuning the gridcircuits of each of said stages. 3

8. A tuned radio receiving apparatus com prising an electron discharge tube having a grid circuit and a plate circuit, means for tuning the grid circuit, and mechanism for said neutralizing neutralizing the capacity coupling between the grid andplate circuits, said neutralizing mechanism comprising a transformer having primary and secondary coils, the primary'coil of which is connected between the plate and filament systems of the electron discharge tube, and 'a neutralizing capacity and an auxiliary coil connected in seriesbetween the grid and the said primary coil of said transformer for impressing a charge on the grid opposite in sign to the a charge carried to the grid from the output circuit through the grid-plate capacity, said auxiliary coil and primary coil being coupled together with a coupling less than unity and being separately and closely coupled to the transformer secondary coil.

9. ,A tuned radio frequency amplifier comprising an electron discharge tube having a grid circuit and a plate circuit, means for tuning the grid circuit, a coil for coupling one of'said circuits to a third circuit, and mechanism for neutralizing the capacity coupling between the 'grid' and third circuits due to the capacity between the grid and plate electrodes, said neutralizing mechanism comprising a second coil connected between one of the electrodes and the filament system and an auxiliary coil and a neutralizing capacity connected in series between the other of these electrodes and the filament system, said auxiliary coil being coupled electromagnetically with a coupling less than unity to said second coil and said auxiliary. and second coils being coupled separately and closely to saidfirst coil.

10. A tuned radio frequency amplifier comprising an electron discharge tube having a grid circuit and a plate circuit, a tuned circuit coupled to one of said tube circuits, and mechanism for neutralizing the capacity coupling between the other of said tube circuits and the tuned circuit via the capac ity between the grid and plate electrodes,

said neutralizing mechanism comprising a coil connected between one of the electrodes and the filament system and an auxiliary coil and a neutralizing capacity connected in series between the other of the electrodes and the filament system, the said coil and auxiliary coil being closely coupled electromagnetically to said tuned circuit and being coupled together closely electromagnetically said coil and auxiliary coil being coupled together electromagnetically with a coupling substantially less than unity and being separately and closely coupled to said tuned circuit.

12. A tuned radio frequency amplifier comprising an electron discharge tube having a grid circuit and a plate circuit, a tuned circuit coupled to the said plate circuit, and mechanism for neutralizing the capacity coupling between the said grid and tuned circuits via the capacity between the grid and plate electrodes, said neutralizing mechanism comprising a coil c0nnectedbetween the plate electrode and the filament system and an auxiliary coil and a neutralizing capacity connected in series between the .grid electrode and the filament system, the

said ,coil and auxiliary coil being coupled together electromagnetically with a coupling substantially less than unity and being separately, and equally coupled electrmnagI- neticall-y to said turned circuit;

13. A tuned radio receiving apparatusf comprising'a'n electron discharge tube hav-" ing a grid circuitand a' plate circuit, a tuned circuit having. an air cored inductance coil coupled to said plate circuit, and mechanism for neutralizing the coupling between the said grid and tuned circuits via the capacity 'between the grid and plate electrodes, said 7 neutralizing mechanism comprising a coil connected between the plate electrode and .the filament system and an auxiliary coil and a neutralizing capacity connected in series between the grid electrode and the filament system, the said coil and auxiliary coil being coupled to said air cored inductance coil to produce oppositereaction charges on said plate electrode and said neutralizing capacity.

14:.- A tuned radio receiving apparatus comprising an electron discharge tube having a grid circuit and a plate circuit, a tuned circuit having an air cored inductance coil coupled to said plate circuit, and mechanism for neutralizing the coupling between the said grid and tuned circuits via the capacity between the grid and plate electrodes, said neutralizing mechanism comprising a coil connected between the plate electrode and the filament system and an auxiliary coil and a neutralizing capacity connected in series between the grid electrode and the filament system, the said coil and auxiliary 'co1l being coupled together closely but with cou- .7

pling less than unity and being coupled equally-to said air cored inductance coil to.

produce opposite reaction charges on said plate electrode and said neutralizing capacity.

15. A tuned radio frequency amplifier comprising an electron discharge amplifier tube having a grid circuit and a plate circuit, a tuned circuit having an air cored inductance coil coupled to one of these circuits, and mechanism for neutralizing the coupling between the other of these circuits and the tuned circuit via the capacity \between-the grid and plate electrodes, said neutralizing mechanism comprising acoil connected between one of the electrodes and the filament system and an auxiliary coil and a neutralizing capacity connected in series between the other of the electrodes and the filament system, the said coil and auxiliary coil being ltlll coupled together closely and being coupled tube having a grid circuit and a plate circuit, a tuned circuit having an air cored inductance coil coupled to one of these circuits, and mechanism for neutralizing the coupling between the other of these circuits and the tuned circuit via the capacity be- ;tween the grid and plate electrodes, said neutralizing mechanism comprising a coil connected between one of the electrodes and the filament system and an auxiliary'coil and produce opposite reaction charges on said a neutralizing capacity connected in seriesj one of the electrodes and said neutralizing between the other of the electrodes and the capacity. v 1 filament system, the said coil and auxiliary In testimony whereof, I have signed my 5 coil being coupled together closely but with name to this specification, this 14th day of a coupling less than unity and being coupled March, 1927. p equally to said air cored inductance coil to LESTER L. JONES. 

